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Build on top of foam? Are you crazy? The compressive strength of the XPS foam I used is 25 psi. That works out to 3600 lb. per sq. ft. (psf). The default bearing capacity of soil in the IRC, which I used to calculate footing sizes, is 1500 psf. In essence, the foam has more than twice the assumed bearing capacity of the earth below. Foam comes in a variety of densities, so check with the manufacturer to be sure that what you use can handle the load. Calculating the amount of foam I needed was simple: length × width = sq. ft. of foam. Dividing that by 32 sq. ft. (the size of a 4x8 sheet of foam) gives the number of sheets. about half the cost of 2x12 boards. Some of it I ripped into 4-in. ribs to strengthen and straighten the forms. Forming was simple–I just outlined the slab perimeter with the poplar. Using strings to be sure the forms were straight, I braced them about every 4 ft. to stakes driven into the dirt at the edge of the excavation. This was overkill, but if you’ve ever heard the awful cracking sound a form filled with wet concrete makes when it lets go, you know overkill is the way to do it. Once the forms were up, I covered the foam with 6-mil plastic. Laying down foam is dirt simple. Make sure the foam fully contacts the gravel. Foam sheets don’t need to fit tightly. Gaps can be filled with scrap pieces and spray foam. The ASCE document called for two rows of #4 rebar in the footings, and I added a rebar grid to reinforce the slab. I laid this out before the pour, then cut and bent the pieces as needed. I placed the pieces in the concrete as the pour progressed so there would be less to trip on as my helpers and I placed and screeded the concrete. Where two pieces of rebar meet, they should overlap by at least 16 bar diameters. The diameter of #4 rebar is 1⁄2 in., so I overlapped the pieces at least 8 in. 56 FINE HOMEBUILDING No different from pouring any other slab Concrete is available in a variety of strengths, with 2500 psi being the code minimum for residential footings. For a few dollars more per yard, I ordered 3500 psi. Because the garage would be unheated much of the time, I also ordered air-entrainment, which intro duces tiny air bubbles into the mix and reduces the chance of spalling caused by freeze-thaw cycles. That would be more impor tant for a driveway than for an interior slab, but it only cost a buck a yard more. I always schedule concrete for a slab for first thing in the morning. If it’s cold, you have all day for it to set up. If it’s hot, you do the hardest work in the coolest part of the day. After finishing the slab, I covered the new concrete with plastic. To develop concrete’s full strength, it must be kept wet for as long as the chemical reaction between the portland cement and the water that causes it to cure continues. The curing slows as the reaction consumes the components, so